Shallow gas detection – why HRS, why 3D, why not HRS 3D?

Detection of shallow gas, and mitigation of this hazard while drilling, is the most significant and critical aspect of geohazard site surveys. Case studies show that the interpretation of shallow gas from seismic data, although well understood, is by no means unambiguous. In spite of the developments in acquisition and processing techniques over the last 20 years, which have improved the quality and interpretability of high resolution seismic data, the industry standard is still limited to the acquisition of 2D datasets. This paper compares the suitability of 2D and 3D exploration seismic data with high resolution seismic (HRS) data, and shows the advantages of better vertical and horizontal resolution that can be achieved with the use of HRS 3D data. Conventional 2D exploration seismic data are inadequate to identify shallow gas because of limited vertical resolution. In shallow water, the highest quality 3D exploration seismic data will not suffice either, because of the high angles of incidence for shallow reflection events. In deep water, 3D exploration data may be used for geohazard evaluation provided the dataset is of sufficient quality, but the best dataset for identification of shallow gas would always be an HRS 3D dataset.